METHANE PRODUCTION AND RECOVERY USING
POTATO WASTE SOLUBLES AND SOLIDS
Melbourne L. Jackson, Professor
Department of Chemical Engineering
University of Idaho
Moscow, Idaho 83843
BASIS FOR DESIGNS
The production and recovery of methane gas utilizing various potato conversion wastes
in either soluble or solid form is of potential interest as a partial fuel source for processing
plants. Such wastes may occur at temperatures ranging from about 25 C to as high as 55 C.
The form and concentration of the waste and the temperature may dictate the choice of
one of several possible treatment processes which are variously designated as conventional
(holding tanks), contact (recycle of cell solids), and filter (packed bed) reactors. To this
may be added a recent concept of an expanded bed reactor. A first design approach was
employed to approximate capital and operating costs with an evaluation of the internal
rates of return on investment and payout times. The fraction of the energy required by a
processing plant which could be provided by the gas produced was of interest as a potential
supplemental source of energy in the event that natural gas supplies are reduced or that
prices of this fuel continue to escalate rapidly.
An anaerobic growth process has advantages and disadvantages in comparison to an
aerobic process for fermentation type waste treatment. The metabolic product methane
may be useful as a fuel source for conditions which favor the growth of the acid forming-
methanogenic symbiotic organisms. No molecular oxygen need be provided, the quantity
of cells produced is much less (by one-seventh), with nutrient requirements reduced accordingly and may not need to be added. Loading rates can be high which permits processing
of wastes having high organics providing toxicity is not limiting; the much slower growth
rate from lower kinetic values requires larger holding equipment, and cell retention times
must be longer, days instead of hours. Recovery of the gases produced may require storage
facilities unless the gas can be used immediately. Processes must provide longer cell retention times such as by recycle of solids or by use of an attached growth arrangement. Higher
temperatures are of interest to increase the conversion rates because the metabolic processes
increase substantially, and predictably, with an increase in temperature. Wastewater at
elevated temperatures can possibly be processed at effluent temperatures without the
added cost or need for cooling water. Many accounts of anaerobic processes appear in the
literature, most on a laboratory scale. Only a few applications to large or full-scale plants
will be considered here.
Most anaerobic processes operating to date have employed the two-tank system, termed
here as conventional, usually without mixing in the first tank, with the second tank employed to settle solids for separation. The gasses are usually burned either to supply heat to
the reactor (digesters) or for disposal in the open air. The residual organic food concentration in the second tank must be low to minimize gas formation and floating of the solids.
Provision is made for withdrawal of solids from both the bottom and the top scum layer,
with clearer effluent for discharge taken from intermediate levels. A conventional two-tank
system usually is not employed for either high processing rates, high flows, or for the recovery of the fuel gas.
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